Many modern portable computers, including laptops and personal digital assistants (PDAs), have a built-in PCMCIA slot (i.e. socket) for accepting another electronic device packaged in a PC (personal computer) card conforming to the PCMCIA standard. PCMCIA (Personal Computer Memory Card International Association) is an organization that was formed in 1989 with the original purpose of developing and promoting standards for PC cards that could provide additional memory resources for the host computer. This purpose has since expanded to encompass other types of electronic devices such as, for example, PC card wireless modems that are capable of communicating with a remote device over a wireless link.
FIG. 1 shows a block diagram of a wireless-communication-enabled computer system 10 comprising a host computer 100 and a PC card wireless modem 102. PC card wireless modem 102 includes an antenna 104 for transmitting/receiving radio frequency (RF) signals to/from a remote device over a wireless network. PC card wireless modem 102 also includes various input/output (I/O), power and ground terminals 106, which are arranged according to the PCMCIA standard. Host computer 100 communicates with PC card wireless modem 102 via a PCMCIA interface 108, when terminals 106 are plugged into a PCMCIA slot 110 of host computer 100. PCMCIA interface 108 not only provides a communication means, it also includes power and ground terminals that couple a power supply of host computer 100 to the power and ground terminals of PC card wireless modem 102.
FIG. 2 shows a wireless communications system 20 that includes a wireless-communication-enabled host computer, such as that shown in FIG. 1. A host computer 200, shown as a laptop, has a PCMCIA slot 202 with a PC card wireless modem 204 plugged into the slot 202. PC card wireless modem 204 has an antenna 206 that transmits/receives RF signals modulated by data and voice information to/from a base station 208 over a wireless link 210. Base station 208 transmits/receives voice modulated signals to/from a mobile switching center 212, which communicates with a remote device (e.g. a telephone) over the PSTN (Public Switched Telephone Network) 214. Base station 208 also transmits/receives data modulated signals to an ISP (Internet Service Provider) Server 216. ISP server 216 transmits/receives data to/from a gateway/router 218, which sends/receives the data to/from a remote over the Internet 220.
Base station 208 in FIG. 2 may be associated with any number of networks. For example, it may be associated with a pager network or a wireless communications network used by cellular telephones. One particular cellular telephone network that is in common use in Europe and of increasing use throughout the rest of the world is GSM (Global System for Mobile communications). Besides functioning as a voice network, GSM is becoming particularly attractive to users and developers of wireless-communication-enabled computers, such as the ones described above in connection with FIGS. 1 and 2. A large reason for this is that GSM supports packet-switched data protocols like GPRS (General Packet Radio Service). Packet-switched data makes more efficient use of available bandwidth and is typically faster than traditional circuit-switched data protocols. GPRS also supports the Internet Protocol (IP), thereby allowing users of a computing device with a GPRS-compatible PC card wireless modem to gain access to the Internet.
GPRS operates by allocating timeslots for packet data transmissions upon a request by a user and freeing up timeslots when not required by the user. The wireless-communication-enabled computer system breaks down an Internet TCP/IP (Transmission Control Protocol/Internet Protcol) data message into data packets. When the data is ready to be sent, the network assigns timeslots on a channel for the transmission. The GPRS-compatible modem transmits the data packets in the assigned timeslots to the cellular base-station where the packets are reassembled into the original TCP/IP data message and finally passed to the Internet for transport to the destination.
Timeslots in a GSM/GPRS network are delineated similar to that in TDMA (Time Division Multiple Access) technology. Each channel is divided into eight timeslots, which are then allocated to different requesting users. More than one timeslot may be requested and allocated to increases the rate at which the modem is permitted to transmit data. However, when this is done more power is demanded from the host power supply, which explained above, functions as the power source of the modem. When the current increases, the voltage supplied by the host power supply tends to drop, due to the internal resistance of the power supply. If the current demanded by the modem exceeds that capable of being delivered by the host supply, the host power supply may be damaged and/or the modem may shut down or reset. Unfortunately, the PC card wireless modem does not know what the supplying capability of the host power supply is.
One solution proposed to avoid the current overdraw problem is to include a supplemental battery pack on the PC card. This approach is undesirable, however, as it increases the size of the PC card, making it more bulky and less popular with users. Another solution would be to simply reduce or limit the RF power of the modem. This approach is also undesirable, however, as it reduces the range of operation of the modem and also unnecessarily sacrifices performance of systems in which the power supply is not the limiting factor.